No one looking at a salmon or a trout would mistake it for a bird, a snake, a frog, or a dog. Salmon and trout meet the definition of a fish. Fish are generally defined as aquatic vertebrates characterized by scales, fins, and pharyngeal gills.
(Fish species are so varied, exceptions exist for each characteristics. Lamprey and Hagfish, the most primitive vertebrate fish, have no mouth, no backbone, and no fins. Sharks have no bones. Their entire endoskeleton is made of cartilage. [Endo comes from a Greek word, endon. It means within or inner. Skeleton comes from skeletos, another Greek word. It means dried up.])